Esters of polyhydric alcohols, also called polyol esters, find a wide range of varying uses in industry, for example as plasticizers or lubricants. The selection of suitable starting materials allows the physical properties, for example boiling point or viscosity, to be controlled, and the chemical properties, such as hydrolysis resistance or stability to oxidative degradation, to be taken into account. Polyol esters can also be tailored to the solution of specific performance problems. Detailed overviews of the use of polyol esters can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, 1985, VCH Verlagsgesellschaft, vol. A1, pages 305-319; 1990, vol. A15, pages 438-440, or in Kirk Othmer, Encyclopedia of Chemical Technology, 3rd edition, John Wiley & Sons, 1978, vol. 1, pages 778-787; 1981, vol. 14, pages 496-498.
The use of polyol esters as lubricants is of great industrial significance, and they are used particularly for those fields of use in which mineral oil-based lubricants only incompletely meet the requirements set. Polyol esters are used especially as turbine engine and instrument oils. Polyol esters for lubricant applications are based frequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2,2,4-trimethylpentane-1,3-diol, glycerol or 3(4),8(9)-dihydroxymethyltricyclo[5.2.1.02,6]decane, also known as TCD alcohol DM, as the alcohol component.
Polyol esters are also used to a considerable degree as plasticizers. Plasticizers find a variety of uses in plastics, coating materials, sealing materials and rubber articles. They interact physically with high-polymeric thermoplastic substances, without reacting chemically, preferably by virtue of their dissolution and swelling capacity. This forms a homogeneous system, the thermoplastic range of which is shifted to lower temperatures compared to the original polymers, one result being that the mechanical properties thereof are optimized, for example deformation capacity, elasticity and strength are increased, and hardness is reduced.
A specific class of polyol esters (they are referred to as G esters for short) contains diols or ether diols as the alcohol component, for example ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of alcohol and acid, optionally in the presence of acidic catalysts, further processes are employed in practice to obtain G esters, including the reaction of diol with acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, preference usually being given to the esterification of diol and acid.
This is because this process can be performed with no particular complexity in conventional chemical apparatus, and it affords chemically homogeneous products. Compared to this, ethoxylation requires extensive and costly technical equipment.
The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is typically performed in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed in the course of the reaction ensures that the equilibrium is shifted in accordance with the law of mass action to the side of the reaction product, i.e. of the ester, which means that high yields are achieved.
Comprehensive information regarding the preparation of esters of polyhydric alcohols, also including esters of ethylene glycols and fatty acids, and regarding the properties of selected representatives of these compound classes can be found in Goldsmith, Polyhydric Alcohol Esters of Fatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters of diethylene glycol, of triethylene glycol and of polyethylene glycols are prepared at temperatures of 130 to 230° C. over reaction times of 2.5 to 8 hours. Suitable catalysts mentioned for the esterification of polyhydric alcohols are inorganic acids, acidic salts, organic sulfonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed with the aid of an entraining agent, for example toluene or xylene, or by introducing inert gases such as carbon dioxide or nitrogen.
The production and the properties of fatty acid esters of the polyethylene glycols are discussed by Johnson (edit.), Fatty Acids in Industry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and a series of preparative hints are given. Higher diester concentrations are achieved by the increase in the molar ratio of carboxylic acid to glycol. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing through an inert gas, or performing the reaction under reduced pressure in the presence of a desiccant. When the addition of catalysts is dispensed with, longer reaction times and higher reaction temperatures are required. Both reaction conditions can be made milder by the use of catalysts. In addition to sulfuric acid, organic acids such as p-toluenesulfonic acid and cation exchangers of the polystyrene type are the preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from U.S. Pat. No. 2,628,249, color problems in the case of catalysis with sulfuric acid or sulfonic acid can be alleviated when working in the presence of activated carbon.
Further metallic catalysts used to prepare polyol esters are also alkoxides, carboxylates or chelates of titanium, zirconium or tin, for example according to U.S. Pat. No. 5,324,853 A1. Such metal catalysts can be considered as high-temperature catalysts, since they achieve their full activity only at high esterification temperatures, generally above 180° C. They are frequently added not at the start of the esterification reaction, but after the reaction mixture has already been heated up and has reacted partly with elimination of water. In spite of the relatively high reaction temperatures and relatively long reaction times required compared to the conventional sulfuric acid catalysis, crude esters with a comparatively low color number are obtained in the case of catalysis with such metal compounds. Common esterification catalysts are, for example, tetraisopropyl orthotitanate, tetrabutyl orthotitanate, tetrabutyl zirconate or tin(II) 2-ethylhexanoate. Further processes for preparing polyol esters are discussed, for example, in DE 10 2009 048 771 A1, DE 10 2009 048 772 A1 and DE 10 2009 048 775 A1. In these processes, the crude ester obtained is subjected to a steam treatment in the course of workup.
It is likewise known that treatment with a peroxidic compound can be undertaken during the process of workup of the crude ester obtained after the esterification stage, in order to improve the color number of the polyol ester (DE 10 2009 048 773 A1). An analogous process using ozone or ozone-containing gases for lightening the color of polyol esters is described in DE 10 2009 048 774 A1. What is common to both processes is that the oxidative treatment is followed directly, without further intermediate steps, by a steam treatment. Advantageously, over the course of the steam treatment, excess peroxidic or ozone-containing compounds are destroyed and water introduced is removed.
However, the stream removed in the steam treatment of the crude ester contains substantial amounts of the desired polyol ester together with a number of further secondary components. In general, the stream removed in the course of steam treatment, which can also be regarded as a secondary stream, based on the organic component, contains 1%-30% by weight of monoester, 40%-80% by weight of polyol ester and, as the remainder to 100% by weight, secondary components such as starting carboxylic acid and esters thereof, low boilers and high boilers.
Since the content of polyol ester in the secondary stream removed with steam is comparatively high, there is a need for a process for recovering a product stream enriched with polyol esters from said secondary stream from polyol ester preparation and recycling it into the process for polyol ester preparation. The recovery of these additional amounts of polyol ester improves the raw material efficiency of the overall reaction and distinctly increases the capacity of the production plant with the same plant configuration without costly capital investment.